Is it time to re-think the T-Slot?

“If you’re not making chips you’re not making money”

Okay, here’s a typical scenario: A salesperson walks into a typical machine shop one day to run a 3” face mill test.  He loads the inserts onto the cutter and runs the test.  Throughout the process he’s crunching numbers.  He’s calculating how much faster the SFM is on his cutter is compared to the competitor. He’s calculating the MRR of his inserts vs. the competition.  At the end of the test he proudly presents his findings to the shop manager.

1818 Eli Whitney Milling Machine

1818 Eli Whitney Milling Machine

“Well, sir it looks like by using my cutter and inserts your cost per edge on each insert will go down by X% and at the end of the year, because of your tool life increases you should save X thousands of dollars!  Do you want to buy my cutter?”

The shop manager says “Boy that’s great but this is only one of my two big jobs and I want to get you to test on my other production job.  Can you come back tomorrow and we can run that cutter on the other parts?”

The salesperson dutifully bows his head and promises to return the next day.  A typical cutting tool salesperson. A typical day.

How could the situation have been different?  When the salesperson walked into the shop he could have stepped back and taken a look around the place.  He would have noted that, as with most job shops, about half of the machines in the facility where sitting idle, waiting for something to be set up, or changed over.  The salesperson would have recognized that for the customer to be successful and grow they would need to be constantly feeding the machines material and spewing chips into the chips bins as fast as possible.  The salesperson would have recognized that the face mill cutter was only one aspect of the entire operation.  The salesperson would have observed, based upon their own experience of walking into thousands of shops, what kind of roadblocks where preventing the customer from being more productive.

So when the shop manager said ““Boy that’s great but this is only one of my two big jobs and I want to get you to test on my other production job.  Can you come back tomorrow and we can run that cutter on the other parts?” The salesperson would have responded;

“Well golly gee Wally why can’t we set it up and run it now?”

Inevitably the shop manager would respond with something like “I need to changeover for the other job and it’s won’t be in the machine until tomorrow.” “Oh,” says the salesperson, “that makes sense. I’ll see you tomorrow!”

If the salesperson where doing their job they would have said

“Wait a minute dude! How long does it take you to changeover your jobs?  I’m sitting here talking about saving seconds per part and you’re talking about hours of downtime!  Maybe we’re both stepping over dollars to pick up pennies!  Do you want to talk about set up time and changeover reduction?”

In most shops the spindle optimization rate is running around 8-15%.  Companies that have implemented Lean are pushing 80-85% spindle optimization rates: their machines are in the cut 80-85% of the available work day.

Cutting tool technology has improved, machine tool technology has improved but…

“If you always do, what you’ve always done, you always get what you always got” – David Sandler

The bottleneck

There in lies the problem.  No matter how fast the cutters. No matter how fast you can make chips. You will hit a bottleneck around 80 – 85% spindle optimization.  I’ve been in many LEAN shops and the problem each has identified is “set up and changeover” bottleneck.

So let’s take a look at some of the history.

“Systeme Gribeauval”

In the late 18th century, French General Jean Baptiste Vaquette de Gribeauval suggested that muskets could be manufactured faster and more economically if they were made from interchangeable parts. The concept of interchangable parts was introduced.  I think, at this point in time that everyone understands the benefit of interchangable parts.

 

Iron Planer, circa 1825, Photo courtesy: American Precision Museum

Iron Planer, circa 1825, Photo courtesy: American Precision Museum

Eli Whitney

In 1818 Eli Whitney built his first milling machine which precisely shaped metal parts. “His efficient methods, especially the use of interchangeable parts, revolutionized the small-arms industry, and gradually these production methods were applied to most types of manufacturing.”

1862 The Knee Mill

In 1862 Joseph Brown, later of Browne & Sharpe, began development of the fist “Universal Milling Machine”. “In order to insure firmness in the said carriage it is mounted upon a heavy knee”

1939: Bridgeport Patent:

Bridgeport Patent dwg 1942

Bridgeport Patent dwg 1942

“…that many frequently desired machining operations have heretofore been impossible, or at best have required changing the set up of the work on it’s support table, an operation which greatly slows up production and increases the likelihood of inaccuracy in the finished work.”  April 4, 1939  US Patent 2,275,291  MACHINE TOOL OPERATING AT UNIVERSAL ANGLES IN ALL LOCATIONS

Here’s the problem: Changeover of fixtures. It “greatly slows up the production”   We’ve known about it for years.  But what really has been done?

Muda

The table of the machine has not undergone any significant changes in 70 years. In fact, the table of the machine has not really undergone any changes since the early 1820’s!   Think about that!

“The most dangerous kind of waste is the waste we do not recognize.”  – Shigeo Shingo

Mura

Implementing LEAN  in a milling department bottlenecks at 80-85% spindle optimization due to set up and changover. This is the last bottleneck.

Different Needs for Different jobs

Here’s what we know:

  • Some customers need vises
  • Some customer’s need three jaw chucks
  • Some need magnets
  • Some need Vacuum
  • Some need custom fixtures

Is this the BEST way to mount your fixtures?

ALL OF THEM, for the most part, mount with a T-Slot nut.

So, is this the fastest, most accurate way of mounting workholding?

“Where there is no Standard there can be no Kaizen”- Taiichi Ohno

Let’s state the obvious:

  • No single workholding manufacturer can meet all of the needs of every shop
  • No single builder has offered an alternative solution

The problem is that a t-slot has been the most versatile way to mount workholding because there has been no development of a “standard” for workholding…

It’s only the last turn of a bolt that tightens it – the rest is just movement. ~ Shigeo Shingo

Why do we stick with it?  Well, first off it’s pretty versatile. You can mount just about anything to it.

What’s it cost to make a table with a T-Slot?

But let’s step back.  How much does it cost to produce a t-slot on a milling machine  table for the manufacturer of a 40″ x 20″ table:

Present State

  • Rough Machine: 120 mins
  • Finish Machine: 20mins/slot X 5 slots = 100 min
  • Total Time: 220 minutes
  • Shop Rate est: $100/hr
  • Units /month: 50
  • Machine Time/Month (50 X 220) =11,000 minutes (183.3 hours)
  • Cost per month ($100 X 183.3)= $18,333/month
  • Total Annual Cost: $219,996

Now, WHAT IF we could:

  • Reduce he cost of production of milling machine tool talbles?
  • Increase the z-axis travel without changing ANY other dimensions on the machine (other than mod the table casting)?
  • Reduce the overall table weight of the table to reduce intertial load? (NURBS, look-ahead) – less mass in motion
  • Provide a better, more versatile platform for mounting fixtures?

That would be fantastic right?  But we’ve got to realize that there has to be an economic incentive for the builders of machine tools to see the advantage of producing tables for a different kind of specification.

Interchangable parts

What all industry discovered in 1825 was that making parts interchangable was a very good thing.  It led to the development of Mass Production. However, as we moved to this ‘standard’ we failed to create a ‘standard’ for fixturing.  This made sense.  Frankly, there where just far too many applications and fixture designs to come up with a universal mounting system.

But let’s pretend that we had a universal mounting system for base/foundation level fixtures.  How would that change the cost of production of a machine tool table?

Future State:

  • Rough & Finish machine: 30minutes
  • Total Time: 30 minutes
  • Shop Rate est: $100/hr
  • Units /month: 50
  • Machine Time/Month (50 X 30) =1500 minutes (25 hours)
  • Cost per month ($100 X 25)= $2,500/month
  • Difference ($18,333 – $2500=): $15,833 /month Saved
  • Annual savings: $189,996 + Opportunity cost gains from productions time reduction or $379,000

Ok, so we know that the builder’s would be all in if they could save money in production, increase the envelope capacity of the machine, and reduce load on the ways, lead screw, linear guides, etc.  That makes economic sense and it’s something that customers would probably like. (and it makes the cost accountants happy too which is always a bonus)

Jergen's Ball Lock

The Problem: Interchangable fixture components

The real crux of the problem is the that there are so so many manufacturer’s of fixture components.  There is no mounting standard.  As a first step we need a foundation.  There exist two foundation level systems that have interchangability between them (that I know about):

  • Jergen’s Ball Lock
  • Modern’s mPower

That is the foundation.  Again, there may be other systems but these are the only FOUNDATION level systems that I have seen.

From that point forward nothing is interchangable.  But, WHAT IF… the workholding manufacturers’ actually sat down and agreed upon a standard mounting location for their baseline fixtures?

What if they recognized that it would be impossible to change all of their products to a standard location pattern overnight but could slowly make fixturing compatible?  Say, over a 20 year period.

What if, they came up with a compatibility agreement and slowly implemented product changes to comply with this standard?

And what if consumers knew which products where compatible by looking for a universal logo next to the product name that let them know that a particular product was to the “compatibility” standard?

You could agree on a name, say the Clamping Compatibility Consortium, call it “3C’ for short and put a “UL” style stamp on the products that where “interchangable”

It would probably require someone like Caterpillar to get behind it and push the ‘standard’ forward the way the they pushed the CAT tapered toolholders forward….

Until then, I suppose, we shall have to live with 85% spindle optimization.

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